At present, LMI type is a very important configuration between a DTE and a Data Circuit Equipment (DCE) whose communication is implemented with Frame Relay (FR) technique. The so-called LMI is a protocol used for monitoring Permanent Virtual Circuit (PVC) status. In PVC environment, it is necessary for both the network equipment and the subscriber equipment to know current PVC status, i.e. whether the PVC is usable. Moreover, the same kind of LMI protocol must be configured at both of DTE and DCE in practice to guarantee normal communication. There are three kinds of LMI protocols used in prior frame relay links: Q.933 Appendix A of International Telecommunications Union-Telecommunications Standardization section (ITU-T), T1.617 Appendix D of American National Standards Institute (ANSI) and CISCO LMI standard.
In User-Network Interface (UNI) of frame relay, PVC status at the DTE is exclusively determined by the DCE which is responsible for informing DTE with all PVC status in UNI. PVC status in the DCE is determined by network devices. Procedure of obtaining current PVC status at DTE will be described as follows. The DTE sends a STATUS ENQUIRY message which is usually used for verifying link integrity to the DCE at interval of T391, after N391 STATUS ENQUIRY messages have been transmitted, a STATUS ENQUIRY message for enquiring full status will be sent. After receiving a STATUS ENQUIRY message, the DCE will respond with a STATUS message to report current information about PVC. Through polling mechanism of question-answer process between DTE and DCE, the DTE can obtain current status of all PVCs in time. Here, T391 and N391 are two variables defined in ITU-T Q.933 Appendix A. T391 refers to time intervals for enquiring status and sending link integrity message to DCE by DTE, the scope of T391 is 5˜30 seconds and the default value of that is 10 seconds. N391 represents the cycle for enquiring status and sending link integrity message to DCE by DTE, the scope of N391 is 1˜255 and the default value of that is 6, which means every 6 times sending operations for link integrity message will be accompanied with one sending operation for STATUS ENQUIRY message for enquiring full status.
In Network-Network Interface (NNI) of the frame relay, PVC status is exchanged regularly with LMI protocol between both sides of the network devices. Unlike the process in UNI, each side of the network devices sends enquiry messages to the opposite side, and responds after receiving the enquiry messages from the opposite side.
Protocol rules of the LMI mainly include four parts: adding notification of PVC; deleting detection of PVC; notification on usable (activated) or unusable (inactivated) status of the configured PVC; and link integrity verification.
As for the three kinds of available LMI protocols, they have the almost same rules, with slightly difference in frame format. In Q.933 Appendix A, STATUS or STATUS ENQUIRY messages are transmitted through the virtual circuit with DLCI=0. STATUS ENQUIRY message is used for enquiring status and link integrity of the PVC, while STATUS message is used for answer STATUS ENQUIRY message to inform PVC status or link integrity verification. As shown in Table 1 and Table 2, the STATUS message and STATUS ENQUIRY message comprise the following elements respectively, wherein, length is measured in byte, and values of the elements are determined by the protocol according to different message types.
TABLE 1Format of the STATUS messageName of the elementsLength (Byte)Protocol discriminator1Call reference1Message type1Report type3Link integrity verification4PVC status5-7
TABLE 2Format of the STATUS ENQUIRY messageName of the elementsLength (Byte)Protocol discriminator1Call reference1Message type1Report type3Link integrity verification4
Just like ITU-T Q.933, the STATUS ENQUIRY and response messages are also transmitted through the virtual circuit with DLCI=0 in ANSI T1.617 Appendix D. The difference between ITU-T Q.933 and ANSI T1.617 Appendix D lies in that an information element is added in LMI message of the ANSI and values of the elements in the messages are constant. In CISCO LMI standard, the STATUS ENQUIRY and response messages are transmitted through the virtual circuit with DLCI=1023, and values of the elements in the messages are constant, but format of PVC status is different from that of Q.933 and ANSI.
The operation process of the LMI protocol will be briefly described as follows:
1) The DTE sends a status enquiry message STATUS ENQUIRY first, and timer T391 begins to time. The interval of T391 is a time interval of each polling process, which means DTE sends a STATUS ENQUIRY in every two T391s. At the same time, the counter V391 of DTE begins to count. When V391<N391, the DTE sends a STATUS ENQUIRY for enquiring link integrity; else when V391=N391, V391 is reset to zero, and the DTE sends a STATUS ENQUIRY enquiring not only link integrity but also all PVC status which is called STATUS ENQUIRY for enquiring full status. N391 defines length of a cycle, and in every two cycles, the DTE sends a STATUS ENQUIRY for enquiring full status. Values of the T391 and N391 can be set manually, or be chosen with default values.
2) After receiving an enquiry message, the DCE answers status enquiry message STATUS ENQUIRY with status message STATUS, and polling confirm timer T392 of the DCE begins to time, the DCE waits for a next status enquiry message STATUS ENQUIRY. If T392 overtimes and no status enquiry message STATUS ENQUIRY has been received, the DCE will record the error, and the number of error times will be added by 1.
3) The DTE reads the received response message STATUS to find out link status and PVC status. The DCE responds for the status that DTE wants to know. If some changes on PVC status happens in the network or there is an added/deleted PVC, no matter the other side enquires PVC status or not, the DCE will reply DTE with status messages of all PVCs, in order to ensure the DTE to know changes of the DCE and update the former records. If timer T391 overtimes and no status message STATUS has been received, the DTE will record the error, and the number of error times will be added by 1.
4) If the number of error times exceeds N392 in N393 events, the DTE or DCE will consider the physical route and all virtual circuits being unusable. Here, N392 and N393 are two variables defined in ITU-T Q933 Appendix A. N392 is an error threshold, the scope of N392 is 1˜10 and the default value is 3; N393 refers to total number of investigated events, the scope of N393 is 1˜10 and the default value is 4. The values of N392 and N393 can be manually set or be chosen with default values.
In the above-mentioned process, while the DTE and the DCE communicate with each other, only when LMI protocol type set at DTE is the same with that set at DCE, the DCE can recognize status enquiry message STATUS ENQUIRY sent from the DTE, and notify the DTE PVC status through STATUS message. If LMI protocol type set at DTE is different from that set at DCE, the DCE can not recognize status enquiry message STATUS ENQUIRY sent from the DTE, and can not respond with a response message, disabling normal communication between both sides.
It can be seen LMI type configuration plays an important role in the whole communication process between DTE and DCE. But LMI protocol type at DTE and DCE must be configured through manual operation or command line in prior art. In this way, it is necessary for the DTE to know the LMI type at DCE in advance, or try to set different LMI protocol types until communication can be implemented between the two sides. But if the DCE supports the above-mentioned three kinds of LMI protocol types simultaneously, then three configuration trials may be needed at the DTE in order to find out a suitable LMI protocol type. In the communication process after the LMI protocol type has been determined, if the current LMI protocol type at DCE changes, all the communications to the connected DTE devices will be interrupted until the LMI protocol types of all connected DTE devices are changed correspondingly. So, system operation and management is inconvenient, normal communication is seriously interfered, and the whole system is unable to work normally due to data lose.